Rising rates of cesarean sections have been blamed on everything from older first-time mothers to almost phobic fears of vaginal delivery.

Now researchers are suggesting C-sections themselves could be blamed for not making childbirth easier.

According to their theory, the routine use of C-sections in modern times may have stopped the female pelvis from evolving to grow wider to accommodate increasingly larger brains and heads of babies, suggesting C-sections may be having an impact on human evolution.

“Compared with other primates, human childbirth is difficult because the fetus is large relative to the maternal pelvic canal,” the team writes in the latest issue of PNAS, the Proceedings of the National Academy of Sciences. “It is a long-standing evolutionary puzzle why the pelvis has not evolved to be wider, thus reducing the risk of obstructed labour.”

Speaking to the BBC, lead author Philipp Mitteroecker, an anthropologist at the University of Vienna, said women with a particularly narrow pelvis wouldn’t have survived childbirth a century ago.

Thanks to C-sections, they do now, but they also “pass on their genes encoding for a narrow pelvis to their daughters,” he said.

According to their analysis, the routine use of surgical births throughout the past decades has led to an evolutionary increase of “fetopelvic disproportion” — a mismatch between the size of a newborn’s head or shoulders and the mother’s pelvis — by 10 to 20 per cent.

That may explain why rates of obstructive labour are “strikingly high,” in the range of three to six per cent worldwide, they said.

In humans, upright walking evolved four to five million years ago, “long before brain size started to increase about two million years ago,” the team writes in PNAS. The bigger brains grew, the bigger the head sizes of newborns.

But these bigger heads had to be delivered through a pelvis that had been adapted for bipedalism. A wider pelvis, the assumption held, would make it harder for women to walk and run. And, according to New Scientist, there are other pressures to keep women’s pelvises narrow: it reduces premature births.

“With evolutionary pressures forcing the fit to be as tight as possible, there is always going to be some fraction of babies … that can’t squeeze through. Thanks to C-sections, these babies no longer die,” the New Scientist says, and their proportion is rising by up to 20 per cent since the surgery was introduced in the 1950s and 60s.

In 2014-2015, 17 per cent of women under 35 delivered their first baby by C-section, compared with 23 per cent of first-time mothers over 35, according to the Canadian Institute for Health Information.

The overall C-section rate for Canada is hovering around 27 per cent.

“It’s not easy to foresee what this will mean for the future of humans and birth,” Mitteroecker told The Independent.

“Disproportion may further increase. But I don’t think that one day every baby needs to be delivered by C-sections.”